People living in Sri Lanka have started to experience large epidemics of dengue hemorrhagic fever (DHF) after 1989. Previous studies from our group indicate that a genetic change in circulating DEN serotype 3 (DEN3) viruses is responsible for the emergence of DHF in Sri Lanka. DEN3 viruses from Sri Lanka are very closely related and belong to genotype III but isolates from before and after the emergence of DHF are genetically distinct. In this proposal we plan cell culture experiments to identify phenotypic differences between DEN3 strains associated with servere and mild disease. Specifically we plan to test two hypotheses about dengue strains and disease severity. The first hypothesis is that post DHF DEN3 isolates are inherently more virulent and are able to infect human cells better than the pre DHF isolates. To test this hypothesis we plan to compare the ability of DEN3 strains to infect and replicate in human monocytes/macrophages and hepatocyte cell lines. The second hypothesis is that pre-exisitng dengue antibody in the population neutralizes the preDHF DEN3 isolates better than the post DHF isolates. We propose to test this hypothseis by comparing the ability of antisera from people with previous dengue infections to neutralize the pre and post DHF DEN3 isolates. These studies will pinpont key differences between DEN3 strains, which may be responsible for variations in human disease severity.